This sentence should be explained word for word, and the meaning of each word should be understood separately, so that you can naturally know the whole meaning.

Pin: to get involved, also refers to dabbling.

The world of mortals: refers to the whole complex society.

It's upside down: upside down, it's wrong.

More: more hope.

It is unrealistic to fly directly from the ground to the sky.

The whole sentence means: when you set foot in the whole complex society, you did something wrong and went the wrong way, and it is unrealistic to expect to get ahead.

This sentence is to describe a person's ideas and practices as unrealistic.